And I'm Robert Siegel. Secretary of State John Kerry says the U.S. won't sit idly by while Russia fans the flames of instability in Ukraine. But so far, U.S. and European sanctions haven't changed Russia's calculations. Kerry blames Russia for failing to calm the crisis. Russia says Ukraine should stop its offensive against separatists in the east. NPR's Michele Kelemen reports that the diplomatic options during these tense days look limited.

MICHELE KELEMEN, BYLINE: The situation in Ukraine seems to be getting more violent by the day. Now, Kerry is raising concerns about the plans by pro-Russian separatists to organize an independence referendum in two eastern Ukrainian cities on May 11th.

SECRETARY JOHN KERRY: This is really the Crimea playbook all over again and no civilized nation is going to recognize the results of such a bogus effort.

KELEMEN: Russia annexed Crimea earlier this year, and the U.S. and Europe responded by imposing targeted sanctions against Russian officials. Kerry says there will be more sanctions if Russia doesn't help calm the situation, as it promised in talks last month. He was speaking after a meeting with the European Union's top diplomat, Catherine Ashton.

CATHERINE ASHTON: Any further steps that destabilize the situation in Ukraine would lead to additional and far-reaching consequences for our relations in a broad range of areas.

KELEMEN: The EU and the U.S. are trying to leave the door open for diplomacy. Kerry says he'll be in Europe next week to consult with his partners on this. Russia's foreign minister, Sergei Lavrov, says he's open to another round of talks on Ukraine, but only if they include pro-Russian separatists.

SERGEI LAVROV: (Russian spoken)

KELEMEN: Those who protest against the government in the south and the east of Ukraine want their voices heard, Lavrov told reporters in Vienna, adding they want to have an equal voice when it comes to deciding the fate of their own country. He says having them at the table with international players could add value. While the Ukrainians don't like that idea, an expert here in Washington, Matthew Rojansky of the Wilson Center, says this is something the U.S. should explore.

MATTHEW ROJANSKY: Whoever the Russians want to define as the separatist leadership, if you essentially call their bluff or take them at their word that there is such a leadership and let them help to identify that and let them bring those folks to the table as part of what's billed as a comprehensive settlement discussion, well, then at the very least you're talking to the right people.

KELEMEN: Up to now, he says the U.S. and Europe have been tripped up by Russia. While the West accuses Moscow of fueling the conflict and arming and supporting the separatists, Russia tells a very different story. Still, he thinks there is a diplomatic way out because Ukraine can't win a war and Russian President Vladimir Putin doesn't really want one.

ROJANSKY: Although the rhetorical warfare serves a domestic political purpose for Putin, I think fundamentally they're interested in achieving as much as they can below the pain threshold that really starts to unravel Putin's domestic political and economic model.

KELEMEN: What he calls the dribs and drabs approach on sanctions may be hurting Russia's economy, but it isn't changing Russian behavior. Rojansky says for that the U.S. and Europe would need to convince Putin that sanctions on Russia's oil and gas industry could be next. Michele Kelemen, NPR News, Washington. Transcript provided by NPR, Copyright NPR.